Current-to-voltage signal converter

ABSTRACT

The present disclosure provides a current-to-voltage signal converter which may operate at an adjusted voltage. The current-to-voltage converter includes a trans-impedance amplifier which converts a current input into a voltage output. The voltage output may operate around an undesirable predetermined voltage, and must therefore be adjusted in order to make it suitable for any downstream signal processing circuitry, such as an ADC. As such, a subtractor circuit is coupled to the output of the trans-impedance amplifier. At the input of the subtractor circuit, a voltage adjustment circuit is employed, to adjust the voltage input to the subtractor circuit. As such, the input to the subtractor is adjusted between a first predetermined voltage threshold and a second predetermined voltage threshold, and the subtractor circuit may therefore be a low-voltage component.

FIELD OF THE DISCLOSURE

The present disclosure relates to current-to-voltage signal converters.In particular, it relates to a circuit in which the operational voltagemay be adjusted, such that low-voltage components may be used.

BACKGROUND

In many applications it is necessary to convert current signals intovoltage signals. A way to do this is with a trans-impedance amplifier.

Within the technical field of optical sensing it a trans-impedanceamplifier can convert an output current from an optical photodiode to avoltage signal ready to be converted by an analog-to-digital converter(ADC). For some applications the output voltage range from atrans-impedance amplifier is further conditioned to make it suitable forbeing converted by an ADC. For example, by making the dynamic range ofthe signal match the dynamic range of the ADC.

Optical photodiodes are often biased with a voltage reference and theoutput current signal of the optical photodiode will be dependent on thevoltage reference used. Therefore, it is possible to directly adjust thevoltage reference in order to bias the output current signal. However,this can result in excess components and can often impact upon theaccuracy of the representation of the original optical signal if thevoltage reference is not accounted for.

SUMMARY OF THE DISCLOSURE

The present disclosure provides a current-to-voltage signal converterwhich may operate at a reduced voltage. The current-to-voltage converterincludes a trans-impedance amplifier which converts a current input intoa voltage output. The voltage output may operate above a firstpredetermined voltage or below a second predetermined voltage (where thefirst predetermined voltage is greater than the second predeterminedvoltage), and must therefore be adjusted in order to make it suitablefor any downstream signal processing circuitry, such as an ADC. As such,a subtractor circuit is coupled to the output of the trans-impedanceamplifier. At the input of the subtractor circuit, a voltage adjustmentcircuit is employed, to adjust the voltage input to the subtractorcircuit. As such, the input to the subtractor is adjusted between afirst predetermined voltage threshold and a second predetermined voltagethreshold, and the subtractor circuit may therefore be a low-voltagecomponent.

According to a first aspect there is provided a current-to-voltagesignal converter, comprising: a first stage, configured to convert aninput current signal to a first voltage signal; an analog subtractorcircuit comprising an amplifier, wherein the analog subtractor circuitis electrically coupled to the first stage to convert the first voltagesignal into a second voltage signal; and, a voltage adjustmentarrangement, coupled to at least one input of the amplifier, andconfigured such that the voltage at the at least one input of theamplifier is between a first predetermined voltage threshold and asecond predetermined voltage threshold.

The voltage adjustment arrangement can allow for an improvement issystem cost (reduced component size, power) and an improved systemperformance. This can allow the amplifier to be a low voltage amplifierand/or a unipolar amplifier which can lead to further advantages.

A low voltage amplifier operates with a lower supply voltage than anamplifier. This is a relative concept which, for an integrated circuit(IC), depends on the foundry process. In a foundry process, it ispossible to select different voltage ranges and some are lower, some arehigher. An IC can comprise many different discrete voltage levels e.g.20V, 10V, 5V, 3.3V, 1.8V, 1.3V, etc. To differentiate the two types ofamplifiers, an amplifier in an IC can operate with a supply voltagewhich can be the highest voltage level supported by that IC. A lowvoltage amplifier in an IC can operate with a supply voltage lower thanthe highest voltage level supported by that IC, For example, on one ICthe highest voltage level supported may be 10V, therefore a low voltageamplifier may operate with a supply voltage of 5V. In another example,on one IC the highest voltage level supported may be 5V, therefore a lowvoltage amplifier may operate with a supply voltage of 1.8V.

This can lead to accuracy and power advantages for the low voltageamplifier. Thus, a low voltage amplifier can operate at (or have asupply voltage) an IC supported voltage level less than the maximumvoltage level supported. Alternatively, a low voltage amplifier canoperate at (or have a supply voltage) the lowest voltage level supportedby an IC.

Similarly, an amplifier may operate with an asymmetrical supply voltage,such that the positive supply voltage can be the highest voltage levelsupported by that IC and a negative supply voltage which can be 0V. Thereduced dynamic range of this amplifier supplied by an asymmetricalsupply voltage may also lead to accuracy and power advantages.

It is also possible to arrange the voltage adjustment arrangement tobias the voltage level so that the inputs to the amplifier is a positivevoltage value. This allows the amplifier to be supplied by a unipolarsupply voltage i.e. the negative voltage rail is set to electricalground. This enables the system to operate with greater powerefficiency.

Thus the amplifier may be a low voltage amplifier and/or supplied by aunipolar supply voltage. This can improve system performance (accuracy,power efficiency) and reduce cost of the system (reduced surface areaand size).

The voltage adjustment arrangement can comprise a first referencesource, coupled to the at least one input of the amplifier.

The at least one input of the amplifier can comprise a first input and asecond input. The first reference source can be coupled to both thefirst input and the second input, and can be a common mode reference.

The common mode reference can be a voltage or current source.

The first predetermined voltage threshold is greater than the secondpredetermined voltage threshold, wherein the first voltage signal is atleast partially above the first predetermined voltage threshold or atleast partially below the second predetermined voltage threshold.

The amplifier can be a low voltage amplifier and can operate in itslinear region. The amplifier can have a predetermined input voltagerange, and the first predetermined voltage threshold can be less than orequal to the upper end of the predetermined input voltage range, and thesecond predetermined voltage threshold is greater than or equal to thelower end of the predetermined input voltage range. The first and secondpredetermined voltage thresholds may be substantially equal to thepositive and negative supply voltage of amplifier divided by the gain ofthe amplifier, respectively.

The first stage can be configured to operate within a first supplyvoltage range. The amplifier can be an operational amplifier and can beconfigured to operate within a second supply voltage range. The firstsupply voltage range can be greater than the second supply voltagerange. The amplifier can be configured to be further powered by aunipolar supply voltage. The positive and negative supply voltage of theamplifier can be different from the positive and negative supply voltageof the first stage. The dynamic range of the amplifier can be less thanthe dynamic range of the first stage. The voltage range between thepositive and negative supply voltage of the amplifier is less than thevoltage range between the positive and negative supply voltage of thefirst stage.

The input current signal can be generated by a photodiode biased by afirst reference voltage. The first stage can be a trans-impedanceamplifier comprising a voltage input and a current input. The firstreference voltage can be electrically coupled to the voltage input andthe input current signal can be electrically coupled to the currentinput. The second voltage signal can be suitable for supplying an Analogto Digital Converter.

The analog subtractor circuit can further comprise a first, second,third and fourth resistors. The first resistor can be electricallycoupled in series between the first input and the first voltage signal.The second resistor can be electrically coupled to the first input andforms a negative feedback loop based on the output of the operationalamplifier. The output of the operational amplifier can be the secondvoltage signal. The third resistor and the fourth resistor can form apotential divider such that the third resistor can be electricallycoupled in series between the second input and the first referencevoltage, and the fourth resistor can be electrically coupled to a groundstate. The first and third resistors can have substantially the samevalue resistance and the second and fourth resistors can havesubstantially the same value resistance.

The common mode reference can comprise a/the fixed voltage sourceelectrically coupled to a first terminal of a first common mode resistorand a first terminal of a second common mode resistor. The secondterminals of the first and second common mode resistors can beelectrically coupled to the first and second inputs respectively. Thefirst and second common mode resistors can have substantially the samevalue resistance.

The first reference voltage can have a predetermined value, and thecommon mode reference is a fixed voltage source can have a predeterminedvoltage based on the predetermined value.

The voltage adjustment arrangement can comprise a feedback loopconfigured to adjust the voltage of both the first input and secondinput into the amplifier of the analog subtractor circuit. The voltageadjustment arrangement can comprise a feedback amplifier configured toreceive the second input to the amplifier of the analog subtractorcircuit.

The feedback amplifier can be an NMOS transistor. The voltage of boththe first input and the second input into the amplifier of the analogsubtractor circuit can correspond to at least the threshold voltage ofthe NMOS transistor.

The feedback amplifier can be electrically coupled to a first terminalof a first common mode resistor and to a first terminal of a secondcommon mode resistor. The second terminals of first and second commonmode resistors can be electrically coupled to the first and second inputrespectively. The first and second common mode resistors can havesubstantially the same value resistance.

The feedback amplifier can be configured to operate a current mirrorarrangement comprising two mirrored transistors and a controltransistor. The feedback amplifier can be arranged to control thecurrent flow in the control transistor and the two mirrored transistorscan be electrically coupled to the first and second input respectively.The two mirrored transistors can be size matched.

The first stage can include a first reference voltage. The first voltagesignal can be dependent on the first voltage reference. The secondvoltage signal can be independent of the first voltage reference.

According to a second aspect there is provided a transimpedanceamplifier, comprising: a current-to-voltage converter, configured toreceive a first current signal, and to generate a positive voltageoutput; a voltage adjustment arrangement, coupled to thecurrent-to-voltage converter, and configured to enable at least part ofthe current-to-voltage converter to between a first predeterminedvoltage threshold and a second predetermined voltage threshold. Thevoltage adjustment arrangement can comprise a constant voltage/currentsource coupled to a first terminal of a resistor. A second terminal ofthe resistor can be coupled to the current-to-voltage converter.

The voltage adjustment arrangement can comprise a feedback amplifierarranged to actively control the reduction in voltage based on thepositive voltage output.

According to a third aspect there is provided a method of converting acurrent signal to a voltage signal, the method comprising: converting aninput current signal to a first voltage signal using a first stage of asignal converter; converting the first voltage signal into a secondvoltage signal using an analog subtractor circuit comprising anamplifier; and, applying a voltage adjustment arrangement to at leastone input of the amplifier to reduce the voltage at the at least oneinput to the amplifier between a first predetermined voltage thresholdand a second predetermined voltage threshold.

The at least one input of the amplifier can comprise a first input and asecond input. The voltage adjustment arrangement can comprise a commonmode reference coupled to both the first input and the second input. Thecommon mode reference can be a DC source.

The method can further include performing the step of predetermining aDC offset voltage of the first voltage signal. The method can furtherperforming the step of predetermining a voltage or current value of theDC source based on the predetermined DC offset voltage.

The method can further include performing the step of adjusting avoltage or current value at both the first input and the second inputbased on a DC offset voltage of the first voltage signal.

The method can further include performing the step of receiving theinput current signal from a photodiode. The method can furtherperforming the step of applying the second voltage signal to an Analogto Digital Converter.

DESCRIPTION OF THE FIGURES

The present disclosure will now be described, by way of example only, inconjunction with the attached drawings, in which:

FIG. 1 illustrates a circuit diagram of a current-to-voltage signalconverter;

FIG. 2 illustrates a circuit diagram of a current-to-voltage signalconverter with a voltage adjustment arrangement;

FIG. 3 illustrates a circuit diagram of a current-to-voltage signalconverter with a feedback voltage adjustment arrangement with an NMOStransistor;

FIG. 4 illustrates a flow diagram of the method of converting a currentsignal to a voltage signal with a voltage adjustment arrangement;

FIG. 5 illustrates a block diagram of a current-to-voltage signalconverter with a voltage adjustment arrangement;

FIG. 6 illustrates a circuit diagram of a current-to-voltage signalconverter with a feedback voltage adjustment arrangement with an op-amp;and

FIG. 7 illustrates a circuit diagram of a current-to-voltage signalconverter with a feedback voltage adjustment arrangement with a currentmirror arrangement.

DETAILED DESCRIPTION

Certain circuit components produce an output in which the signal is avarying current, as opposed to a varying voltage. For example,photodiodes produce a current-based output signal. Signal processingcircuits, which are used to condition analog signals, for example byconverting them to digital signals, typically require voltage-basedsignals, within a particular voltage range. As such, it is necessary toconvert current-based signals to voltage-based signals, such that theycan be processed. This is typically done by a transimpedance amplifier,as noted above. However, the output of a transimpedance amplifier isnormally outside of the range of suitable inputs for downstream signalprocessing components, such as an analog-to-digital converter. As such,a subtractor circuit is used, to adjust the voltage of the outputsignal. Owing to the voltages typical involved, absent any furtherconditioning, the subtractor circuit would need to be a high-voltagecomponent.

The present disclosure introduces a voltage adjustment mechanism, at theinput to the amplifier of the subtractor circuit. For example, a commonmode voltage may be coupled to both the inverting and non-invertinginputs of the amplifier, via a pair of resistors. As such, the voltageat the input to the subtractor amplifier may be adjusted between a firstpredetermined voltage threshold and a second predetermined voltagethreshold, and the subtractor may then be made from low voltagecomponents. This may reduce the area required by the subtractor, and thecost of the circuit.

FIG. 1 illustrates a current-to-voltage signal converter 1. The input tothe current-to-voltage signal converter 1 is an input current signal(Ipd). Ipd can be the output of a photodiode which is biased by a firstreference voltage (Vref). Vref may be any voltage from a high positivevoltage to a high negative voltage depending on the characteristics ofthe photodiode. The output (Vout2) of the current-to-voltage signalconverter 1 is intended to be suitable for receipt by ananalog-to-digital converter (ADC), such that the optical signal incidentonto the photodiode can be accurately interpreted by a digitalprocessing device.

The current-to-voltage signal converter 1 is comprised of two stages: atrans-impedance amplifier 2; and, an analog subtractor circuit 4. Thecurrent-to-voltage signal converter 1 may be understood as atrans-impedance amplifier itself since it performs the overall functionof receiving an input current signal and outputting an output voltagesignal which can be suitable for receipt by an ADC.

The trans-impedance amplifier 2 comprises a first operational amplifier(op-amp) 6 with a trans-impedance feedback resistor (Rtia) 8 coupledbetween the output and the inverting input of the first op-amp 6. Theinput current signal (Ipd) is coupled to the inverting input of thefirst op-amp 6. The first reference voltage (Vref) is coupled to thenon-inverting input of the first op-amp 6. The output of thetrans-impedance amplifier 2 is a first voltage signal (Vout1):

Vout1=Vref−(Ipd*Rtia)  (1)

The analog subractor circuit 4 is electrically coupled to thetrans-impedance amplifier 2 to convert the first voltage signal (Vout1)into a second voltage signal (Vout2). The analog subtractor circuit 4comprises a second op-amp 10 and four resistors 12 a, 12 b, 14 a, 14 b.The first resistor 12 a is electrically coupled in series between theinverting input of the second op-amp 10 and the first voltage signal(Vout1). The second resistor 14 a is electrically coupled to the outputand the inverting input of the second op-amp 10 to form a negativefeedback loop based on the output of the second op-amp 10. The thirdresistor 12 b and the fourth resistor 14 b form a potential dividercircuit. The third resistor 12 b is electrically coupled in seriesbetween the non-inverting input of the second op-amp 10 and the firstreference voltage (Vref), and the fourth resistor 14 b is electricallycoupled to the non-inverting input of the second op-amp 10 andelectrical ground 16. The first resistor 12 a and third resistor 12 bare selected to have substantially the same value resistance (R1). Thesecond resistor 14 a and fourth resistor 14 b are selected to havesubstantially the same value resistance (R2), The resistance values ofthe four resistors 12 a, 12 b, 14 a, 14 b are selected so that thesecond voltage signal (Vout2) is independent of the first referencevoltage (Vref). However, selecting resistance values R1 and R2simplifies the overall circuit design and may lead to reduced circuitcost (manufacturing time, materials, etc.).

The second voltage signal (Vout2) is the output of the analog subtractorcircuit 4:

$\begin{matrix}{{{Vout}\; 2} = {\left( {{Ipd}*{Rtia}} \right)*\frac{R\; 2}{R\; 1}}} & (2)\end{matrix}$

The analog subtractor circuit 4 can be seen as acting as an analogvoltage shifter. Generally, the first output signal (Vout1) has avoltage range from Vref to Vref-Vin, and the second output signal(Vout2) has a voltage range from 0 to Vin. Where Vin represents thesignal of interest (Ipd) from the input (e.g. from the photodiode) andis Ipd*Rtia. This is a voltage shift of −Vref. However, if Vref is ahigh voltage signal, then the second op-amp 10 must be rated as a highvoltage amplifier because the input of the second op-amp 10 ‘sees’ ahigh voltage signal, i.e. there is a high voltage signal incident uponthe inputs of the second op-amp 10. The voltage signal received by thesecond op-amp 10 may also be a negative voltage depending on the valuesof Vref and Vin. The analog subtractor circuit 4 can also be used toamplify Vin, if necessary.

In Integrated circuit (IC) design, high voltage devices generally have alarge physical footprint (e.g. 4-5 times larger than a low voltageamplifier). They are comparatively very large devices, and have poorperformance in comparison to other lower voltage IC components. Thisleads to a high overall system cost with performance limitations.

Since Vref and Vin can vary in time (i.e. time variant signals), theycan both have associated analog voltage ranges. A circuit which canaccept a large analog voltage range and can reduce the analog voltagerange to a suitable range for low voltage devices, can allow downstreamcomponents to be low voltage devices. Such a circuit can reduce thematerial and performance cost of downstream components and also reduceassociated design efforts.

FIG. 2 illustrates a current-to-voltage signal converter 20 which hasthe same structure as the current-to-voltage signal converter 1illustrated in FIG. 1 (i.e. a trans-impedance amplifier 2 and an analogsubtractor circuit 4) with the addition of a voltage adjustmentarrangement 22.

The voltage adjustment arrangement 22 comprises a common mode voltagesource 24, coupled to the inverting and non-inverting inputs of thesecond op-amp 10 via a first common mode resistor 26 a and a secondcommon mode resistor 26 b. By using a common mode voltage source, thevoltage is applied equally to the inverting and non-inverting inputs ofthe second op-amp 10. The common mode voltage source 24 is a fixedvoltage source (Vcm).

As shown in FIG. 2, the common mode voltage source 24 (Vcm) iselectrically coupled to a first terminal of the first common moderesistor 26 a and a first terminal of a second common mode resistor 26b. The second terminals of the first common mode resistor 26 a andsecond common mode resistors 26 b are electrically coupled to theinverting and non-inverting inputs of the second op-amp 10 respectively.The first and second common mode resistors 26 a, 26 b have substantiallythe same resistance value Rcm. This is because resistors 12 a and 12 bhave substantially the same resistance value R1 and resistors 14 a and14 b have substantially the same resistance value R2. That is to saythat the resistor values are chosen so to eliminate Vref from the outputsignal i.e. any resistor values could be chosen for the resistors 12 a,12 b, 14 a, 14 b, 26 a, and 26 b, as long as the second voltage signal(Vout2) is independent of the first reference voltage (Vref).

Example values are as follows: the input current signal may range fromaround 0 to 5 mA; the first reference voltage (Vref) may range fromaround −2.5V to 2.5V; the trans-impedance feedback resistor 8 (Rtia)depends on Ipd, however, it may be around 1.75V divided-by the maximumIpd value (in amperes); the resistors 12 a, 12 b may be around 40K ohm;the resistors 14 a, 14 b may be around 50K ohm; the resistors 26 a, 26 bmay be around 55K ohm; the voltage at the non-inverting and invertinginput of the second op-amp 10 may range from around 0.5V to 2.5V; and,the common mode voltage source 24 may be around 4V. These values may bechanged depending on the application and/or designer choice. Moreover,it will be apparent that it is the ratio of the values of R1:R2:Rcmwhich may result in the voltage range of the desired output (Vout2).

The voltage adjustment arrangement 22 is arranged to bias the voltagelevel at the inverting and non-inverting inputs into the second op-amp10 so that the analog voltage range that is ‘seen’ by the inputs to thesecond op-amp 10 is between a first predetermined voltage threshold anda second predetermined voltage threshold. By adjusting the inputs to thesecond op-amp 10 to between a first predetermined voltage threshold anda second predetermined voltage threshold, the same overall systemperformance can be maintained and even improved if the second op-amp 10is a low voltage op-amp.

As explained above, by using a low voltage op-amp, an improvement insystem cost (i.e. reduced size, power) can be achieved with animprovement in system performance. If the second op-amp 10 is a lowvoltage op-amp then for correct operation of the analog subtractorcircuit 4, the second op-amp will be operating in its linear region.Therefore, voltage adjustment arrangement 22 may be set so that thesecond op-amp 10 does not to amplify the input signal voltage to beyondits supply voltage, thus causing the input signal to saturate (i.e. theop-amp operating in its saturation region). Even with the addition ofextra circuit component such as the voltage adjustment arrangement 22,by using a low voltage op-amp, then power, size, cost can all be reducedin comparison of the circuit illustrated in FIG. 1.

It is possible for Vout1 to be negative as described previously basedthe value of Vref and Vin (Ipd*Rtia). It is also possible to arrange thevoltage adjustment arrangement 22 to bias the voltage level so that theinputs to the second op-amp 10 is a positive voltage value. This allowsthe second op-amp 10 to be supplied by a unipolar supply voltage i.e.the negative voltage rail is set to electrical ground. This enables thesystem to operate with greater power efficiency. In practice, if thesecond op-amp 10 is supplied by a unipolar supply then the negativesupply voltage will be 0V and the positive supply voltage can be thesame as the positive supply voltage for the first op-amp 6. Therefore,the second op-amp 10 will have half of the supply voltage range as thefirst op-amp 6 if the first op-amp 6 has a negative supply voltage ofthe same magnitude as the positive supply voltage. Thus, the secondop-amp 10 can be a low voltage amp.

Thus the second op-amp 10 may be a low voltage op-amp and/or supplied bya unipolar supply voltage. This can improve system performance(accuracy, power efficiency) and reduce cost of the system (reducedsurface area and size).

The voltage value of the common mode voltage source 24 (Vcm) is a fixedvoltage source having a predetermined voltage. This is determined basedon a predetermined value of the first reference voltage (Vref). Thefirst reference voltage (Vref) may not be a stable voltage source andtherefore may fluctuate during operation. However, if the fluctuationsof the first reference voltage (Vref) are relatively small (i.e. a smallVref voltage range), then the pair of common mode resistors 26 a, 26 bcoupled to Vcm can be used. This can reduce the design complicity butcannot handle a very large voltage range of Vref. Thus, the secondop-amp 10 has a predetermined input voltage range which if exceeded maybegin to saturate the first voltage signal (Vout1). The predeterminedvoltage threshold is therefore less than or equal to the upper end ofthe predetermined input voltage range.

The first op-amp 6 can be a high voltage op-amp and the second op-amp 10can be a low voltage op-amp. The first stage (i.e. the trans-impedanceamplifier 2) can be configured to operate within a first supply voltagerange. The second op-amp 10 can be configured to operate within a secondsupply voltage range. The first supply voltage range can be greater thanthe second supply voltage range. Generally op-amps such as the firstop-amp 6 and the second op-amp 10 have two supply voltage contacts: apositive supply voltage contact and a negative supply voltage contact.The negative supply voltage contact may be 0V, or any voltage less thanthe positive supply voltage. Similarly, the positive supply voltagecontact may be 0V, or any voltage greater than the negative supplyvoltage.

The current-to-voltage signal converter 20 performs the method ofconverting a current signal to a voltage signal, as shown in the flowdiagram of FIG. 4. The method can comprise at least the following threesteps:

-   -   S1. Converting an input current signal (Ipd) to a first voltage        signal (Vout1) using the first stage 2 of the signal converter        20.    -   S2. Applying a voltage adjustment arrangement 22. The voltage        adjustment arrangement 22 comprises a common mode reference        (e.g. common mode voltage source 24 which can be DC source 24)        coupled to both inputs (e.g. inverting and non-inverting inputs)        of the amplifier 10 of the analog subtractor circuit 4. The        voltage adjustment arrangement 22 to adjusts the voltage at the        at least one input to the amplifier 10 between a first        predetermined voltage threshold and a second predetermined        voltage threshold.    -   S3. Converting the first voltage signal (Vout1) into a second        voltage signal (Vout2) using the analog subtractor circuit 4        comprising an amplifier 10 with a first and second input.

In absence of the voltage adjustment arrangement, the first voltagesignal (Vout1) may have a DC offset (Vref) which can cause the firstvoltage signal to vary over a large voltage range. This necessitatesthat the amplifier 10 be a high voltage device in order to accuratelysubtract the DC offset (Vref) from the desired signal (i.e. Ipd*Rtia).

The voltage adjustment arrangement 22 can eliminate the requirement thatthe amplifier 10 be a high voltage device by applying an adjustingvoltage to the at least one input to the amplifier 10. This can shiftand/or scale the DC offset (Vref), therefore the input range to theamplifier 10 is adjusted. Therefore, the amplifier 10 can be a lowvoltage amplifier and/or a unipolar amplifier. This can improveperformance and cost (materials) of the overall current-to-voltagesignal converter 20.

The method can comprise the additional step of predetermining the DCoffset voltage (Vref) of the first voltage signal (Vout1) andpredetermining a voltage (or current value) of the DC source 24 based onthe predetermined DC offset voltage (Vref). Depending on the tolerancesof the amplifier 10 (e.g. low voltage amplifier or otherwise) and thevoltage range of the desired voltage signal (i.e. Ipd*Rtia), it can bepossible to fix the value of the DC source 24 to a fixed voltage. Thiscan reduce circuit complexity.

However, the predetermination of the value of the DC source 24 can befurther based on the expected variation in the DC offset (Vref) so thatduring operation the input to the amplifier 10 does not exceed the firstpredetermined voltage threshold or fall below the second predeterminedvoltage threshold. These predetermined voltage thresholds may be thelimit at which the amplifier 10 no longer operates in its linear regionand begins to operate in its saturation region. Similarly, this is not astrict limit and it may be acceptable depending on the application toexceed this limit a percentage of the time, such that for the majorityof time the second voltage signal (Vout2) is representative/linearlyproportional to the input current signal (Ipd).

FIG. 3 illustrates a current-to-voltage signal converter 30 comprisingthe structure of the current-to-voltage signal converter 1 (i.e. atrans-impedance amplifier 2 and an analog subtractor circuit 4) asillustrated in FIG. 1 with the addition of a voltage adjustmentarrangement 32.

The current-to-voltage signal converter 30 operates in much the same wayas the current-to-voltage converter 20 of FIG. 2 with all of the sameadvantages. However, the current-to-voltage converter 30 of FIG. 3 canfurther operate even when the first reference voltage (Vref) has a largeand unknown voltage range/voltage fluctuations, by virtue of a feedbackloop of the voltage adjustment arrangement 32. The voltage adjustmentarrangement 32 comprises a feedback amplifier 36 arranged to activelycontrol the adjustment in voltage based on the positive voltage output.

The voltage adjustment arrangement 32 reacts to variations in the firstreference voltage (Vref) and can further voltage shift the firstreference voltage (Vref) to ensure that the voltage at the non-invertingterminal of the second op-amp 10 remains substantially consistent. Inaddition, any fluctuations in the first reference voltage (Vref) can bereproduced at the inverting input of the second op-amp 10 (similar tothe non-inverting input of the second op-amp 10) so that the secondop-amp 10 can effectively subtract the first reference voltage (Vref)from the first voltage signal (i.e. Vout1).

The voltage adjustment arrangement 32 comprises a first reference sourcewhich is a common mode current source 34. The common mode current source34 is electrically coupled to the first terminal of the first commonmode resistor 26 a and to the first terminal of the second common moderesistor 26 b. The second terminals of first and second common moderesistors 26 a, 26 b are electrically coupled to the inverting andnon-inverting inputs of the second op-amp 10 respectively. Similarly tothe common mode resistors of FIG. 2, the first and second common moderesistors 26 a, 26 b may have substantially the same value resistancefor the same reasons.

The feedback amplifier can be an NMOS transistor 36, as shown in FIG. 3.The gate of the NMOS transistor 36 is electrically coupled to thenon-inverting input of the second op-amp 10. The source of the NMOStransistor 36 is electrically coupled to the ground plane (or electricalground). The drain of the NMOS transistor 36 is electrically coupled tothe common mode current source 34, the first terminal of the firstcommon mode resistor 26 a, and to the first terminal of the secondcommon mode resistor 26 b.

The transistor 36 in this arrangement can be arranged to adjust itschannel until the gate-to-ground voltage is near its threshold voltage(Vth), which can be in the low voltage range. Put another way, the NMOStransistor 36 can ensure that the voltage of both the non-invertinginput and the inverting input into the second op-amp 10 of the analogsubtractor circuit 4 corresponds to at least the threshold voltage (Vth)of the NMOS transistor 36. The threshold voltage (Vth) can typically bearound 0.7V. This results in the common mode of the signal of interest(i.e. Ipd*Rtia) varying around the biased DC voltage signal (i.e. Vth).The selection of Rtia can be predetermined such that the range of thesignal can remain in the low voltage range, such that the second op-amp10 can be a low voltage device. Moreover, the selection of Rcm can bepredetermined such that the signal remains above 0V (i.e. electricalground), such that the second op-amp 10 can further be a unipolarsupplied device and possibly a low voltage device.

The current-to-voltage signal converter 30 performs the method ofconverting a current signal to a voltage signal. The method can compriseat least the three steps described above in reference to thecurrent-to-voltage converter 20.

The current-to-voltage signal converter 30 can further adjust thevoltage or current value at both inputs to the amplifier 10 based on theDC offset voltage (Vref) of the first voltage signal (Vout1). Theadjustment can be a shifting, scaling, and/or stabilisation of the DCoffset voltage (Vref). The value of the voltage or current value at bothinputs to the amplifier is based on the actual variation in the DCoffset (Vref), via a feedback loop. The feedback loop is configured sothat during operation the input to the amplifier 10 does not exceed thefirst predetermined voltage threshold or fall below the secondpredetermined voltage threshold. These predetermined voltage thresholdsmay be the limit at which the amplifier 10 no longer operates in itslinear region, and begins to operate in its saturation region. This isnot a strict limit and it may be acceptable depending on the applicationto exceed this limit a percentage of the time, such that for themajority of time the second voltage signal (Vout2) isrepresentative/linearly proportional to the input current signal (Ipd).

FIG. 5 illustrates a block diagram of a current-to-voltage signalconverter 40 comprising a first stage 42, an analog subtractor circuit4, and a voltage adjustment arrangement 44.

The first stage 42 is configured to convert an input current signal(Ipd) to a first voltage signal (Vout1). This can be accomplished inmany ways as known in the art, however, a specific example is atrans-impedance amplifier circuit 2 shown in FIGS. 1-3.

The second stage of the current-to-voltage signal converter 40 is theanalog subtractor circuit 4. The analog subtractor circuit 4 comprisesan amplifier 10. The analog subtractor circuit 4 is electrically coupledto the first stage 42 to convert the first voltage signal (Vout1) into asecond voltage signal (Vout2) i.e. it performs at least a voltage shiftoperation.

The voltage adjustment circuit 44 is electrically coupled to at leastone input of the amplifier 10, and configured such that the voltage atthe at least one input of the amplifier 10 is between a firstpredetermined voltage threshold and a second predetermined voltagethreshold. The voltage adjustment circuit 44 can be an open loop system,a feedforward system, or a feedback system.

The analog subtractor circuit 4 can further comprise an input for afirst reference voltage (Vref). The first voltage signal (Vout1) cancomprise a component of the first reference voltage (Vref). For example,the first reference voltage (Vref) may be a (time) varying DC offset.Therefore, the voltage adjustment arrangement 44 can introduce a firstreference source to the input of the amplifier 10. This can (partiallyor fully) scale/offset the first reference voltage (Vref) component ofthe first voltage signal (Vout1). The voltage adjustment circuit 44 canbe designed with predefined knowledge of the first reference voltage(Vref) in order to scale/offset it (i.e. an open loop system e.g. FIG.2), or it can be designed to be reactive to the current/future/pastfirst reference voltage (Vref) in operation (i.e. a closed loop system[feedback or feedforward] e.g. FIG. 3). The voltage adjustment circuit44 allows for an improvement in system cost (reduced component size,power) and an improved system performance. This can allow the amplifier10 to be a low voltage amplifier and/or a unipolar amplifier which canlead to further advantages.

In operation, the current-to-voltage signal converter 40, performs themethod of converting a current signal to a voltage signal. The methodcan comprise at least the three steps described above in reference tothe current-to-voltage converter 20 and the current-to-voltage converter30. The method has the same advantages and benefits as stated previouslywith regards to the current-to-voltage converter 20 and thecurrent-to-voltage converter 30.

FIG. 6 illustrates a current-to-voltage signal converter 50 comprisingthe structure of the current-to-voltage signal converter 1 (i.e. atrans-impedance amplifier 2 and an analog subtractor circuit 4) asillustrated in FIG. 1 with the addition of a voltage adjustmentarrangement 52.

The current-to-voltage signal converter 50 operates in much the same wayas the current-to-voltage converter 30 of FIG. 3 with at least all ofthe same advantages. The difference to FIG. 3 is that the NMOStransistor 36 has been replaced with a third op-amp 54 with a biasvoltage (Vb).

The inverting input of the third op-amp 54 is electrically connected tonon-inverting input of the second op-amp 10. The non-inverting input ofthe third op-amp 54 is electrically connected to the bias voltage (Vb)which performs the same purpose as the threshold voltage (Vth) of theNMOS transistor 36. That is, it results in the signal of interest (i.e.Ipd*Rtia) varying around the biased DC voltage signal (i.e. Vb). Thiscan result in an effective shifting or scaling, or at leaststabilisation, of the varying DC offset voltage (i.e. Vref) of the firstvoltage signal (Vout1). It is worth noting that the output of the thirdop-amp 54 can act as the first reference source, hence a common modevoltage/current source (similar to the common mode current source 34 ofFIG. 3) can be absent. The third op-amp 54 can be electrically coupledto the first terminal of the first common mode resistor 26 a and a firstterminal of a second common mode resistor 26 b.

FIG. 7 illustrates a current-to-voltage signal converter 60 comprisingthe structure of the current-to-voltage signal converter 1 (i.e. atrans-impedance amplifier 2 and an analog subtractor circuit 4) asillustrated in FIG. 1 with the addition of a voltage adjustmentarrangement 62.

The current-to-voltage signal converter 60 operates in much the same wayas the current-to-voltage converter 30 of FIG. 3 with at least all ofthe same advantages. The difference to the current-to-voltage converter30 of FIG. 3 is that the first and second common mode resistors 26 a, 26b have been replaced with a current mirror arrangement 63. The currentmirror arrangement comprises a control (PMOS) transistor 66 and twomirrored (PMOS) transistors 68 a, 68 b. The drain of each mirroredtransistor 68 a, 68 b are electrically coupled to the inverting andnon-inverting inputs of the second op-amp 10 respectively. The source ofeach mirrored transistor 68 a, 68 b and control transistor 66, areconnected together and to a power source 69. The gate of each mirroredtransistor 68 a, 68 b, gate of the control transistor 66, drain of thesecond NMOS transistor 64, and the drain of the control transistor 66,are connected together. The source of the second NMOS transistor 64 iselectrically coupled to electrical ground.

The drain of the feedback NMOS transistor 36 is electrically coupled tothe common mode current source 34, and to the gate of a second NMOStransistor 64. The feedback NMOS transistor 36 can therefore, controlthe current flow through the second NMOS transistor 64. The second NMOStransistor 64 can therefore control the current in the current mirrorarrangement 63. The mirrored current drawn by the two mirroredtransistors 68 a, 68 b controls the voltage at the inverting andnon-inverting inputs of the second op-amp 10 similarly to the pair ofresistors 26 a, 26 b of FIGS. 2,3, and 5. Put another way, the twomirrored (PMOS) transistors 68 a, 68 b can aid in an effective shifting,scaling, and/or at least stabilisation of the varying DC offset voltage(i.e. Vref) of the first voltage signal (Vout1). This can enable thesecond op-amp 10 to be a low voltage op-amp and/or supplied by aunipolar supply voltage. This can improve system performance (accuracy,power efficiency) and reduce cost of the system (reduced surface areaand size).

In a current-to-voltage signal converter 30, 40, 60 which can comprise avoltage adjustment arrangement 32, 42, 62 with a closed loop system andan NMOS transistor 36, the NMOS transistor 36 can be changed to aresistor arrangement or other simple amplifier. It is only useful tohave amplifier functionality. Moreover, in the current-to-voltage signalconverters 30, 40, 50, 60 any mosfet type transistors may be changed forBJT type transistors or any other transistor type without deviating fromthe concept described within this description.

For all of the above designs and circuits, it is possible to add anextra diode from the non-inverting input of the second op-amp 10 toelectrical ground (i.e. anode is connected to ground). This diode canprotect at least one input of the second op-amp 10 such that the secondop-amp 10 always operates within a safe range when thecurrent-to-voltage signal converter 1, 20, 30, 40, 50, 60 is initiallypowered up or if the voltage adjustment arrangement 22, 32, 42, 52, 62cannot change the input voltage to the second op-amp 10 immediately.

An analog subtractor circuit can be modified with a feedback circuit sothat the second op-amp does not ‘see’ a high voltage signal. Thefeedback circuit can have many different implementations and it can alsobe replaced with a feedforward circuit. To protect the low voltageamplifier from this circuit suddenly failing, a passive protectioncircuit can be used.

General

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,”“include,” “including,” and the like are to be construed in an inclusivesense, as opposed to an exclusive or exhaustive sense; that is to say,in the sense of “including, but not limited to.”

The words “coupled” or “connected”, as generally used herein, refer totwo or more elements that may be either directly connected, or connectedby way of one or more intermediate elements. Additionally, the words“herein,” “above,” “below,” and words of similar import, when used inthis application, shall refer to this application as a whole and not toany particular portions of this application. Where the context permits,words in the Detailed Description using the singular or plural numbermay also include the plural or singular number, respectively. The words“or” in reference to a list of two or more items, is intended to coverall of the following interpretations of the word: any of the items inthe list, all of the items in the list, and any combination of the itemsin the list.

It is to be understood that one or more features from one or more of theabove-described embodiments may be combined with one or more features ofone or more other ones of the above-described embodiments, so as to formfurther embodiments which are within the scope of the appended claims.

1. A current-to-voltage signal converter, comprising: a first stage,configured to convert an input current signal to a first voltage signal;an analog subtractor circuit comprising an amplifier, wherein the analogsubtractor circuit is electrically coupled to the first stage to convertthe first voltage signal into a second voltage signal; and a voltageadjustment arrangement, coupled to at least one input of the amplifier,and configured such that the voltage at the at least one input of theamplifier is between a first predetermined voltage threshold and asecond predetermined voltage threshold.
 2. The current-to-voltage signalconverter of claim 1, wherein the voltage adjustment arrangementcomprises a first reference source, coupled to the at least one input ofthe amplifier.
 3. The current-to-voltage signal converter of claim 2,wherein the at least one input of the amplifier comprises a first inputand a second input, wherein the first reference source is coupled toboth the first input and the second input, and is a common modereference.
 4. The current-to-voltage signal converter of claim 3,wherein the common mode reference is a voltage or current source.
 5. Thecurrent-to-voltage signal converter of claim 1, wherein the firstpredetermined voltage threshold is greater than the second predeterminedvoltage threshold, wherein the first voltage signal is at leastpartially above the first predetermined voltage threshold or at leastpartially below the second predetermined voltage threshold.
 6. Thecurrent-to-voltage signal converter of claim 1, wherein the amplifier isa low voltage amplifier and operates in its linear region, and theamplifier has a predetermined input voltage range, and the firstpredetermined voltage threshold is less than or equal to the upper endof the predetermined input voltage range, and the second predeterminedvoltage threshold is greater than or equal to the lower end of thepredetermined input voltage range.
 7. The current-to-voltage signalconverter of claim 1, wherein the first stage is configured to operatewithin a first supply voltage range, wherein the amplifier is anoperational amplifier and is configured to operate within a secondsupply voltage range, and the first supply voltage range is greater thanthe second supply voltage range, and/or the amplifier is configured tobe further powered by a unipolar supply voltage.
 8. Thecurrent-to-voltage signal converter of claim 1, wherein the inputcurrent signal is generated by a photodiode biased by a first referencevoltage; wherein the first stage is a trans-impedance amplifiercomprising a voltage input and a current input, wherein the firstreference voltage is electrically coupled to the voltage input and theinput current signal is electrically coupled to the current input;and/or wherein the second voltage signal is suitable for supplying anAnalog to Digital Converter.
 9. The current-to-voltage signal converterof claim 1, wherein the analog subtractor circuit further comprises afirst, second, third and fourth resistors; wherein the first resistor iselectrically coupled in series between the first input and the firstvoltage signal; wherein the second resistor is electrically coupled tothe first input and forms a negative feedback loop based on the outputof the operational amplifier, wherein the output of the operationalamplifier is the second voltage signal; wherein the third resistor andthe fourth resistor form a potential divider such that the thirdresistor is electrically coupled in series between the second input andthe first reference voltage, and the fourth resistor is electricallycoupled to a ground state; and, wherein the first and third resistorshave substantially the same value resistance and the second and fourthresistors have substantially the same value resistance.
 10. Thecurrent-to-voltage signal converter of claim 3, wherein the common modereference comprises a/the fixed voltage source electrically coupled to afirst terminal of a first common mode resistor and a first terminal of asecond common mode resistor, wherein the second terminals of the firstand second common mode resistors are electrically coupled to the firstand second inputs respectively, wherein the first and second common moderesistors have substantially the same value resistance.
 11. Thecurrent-to-voltage signal converter of claim 8, wherein the firstreference voltage has a predetermined value, and the common modereference is a fixed voltage source having a predetermined voltage basedon the predetermined value.
 12. The current-to-voltage signal converterof claim 3, wherein the voltage adjustment arrangement comprises afeedback loop configured to adjust the voltage of both the first inputand second input into the amplifier of the analog subtractor circuit,wherein the voltage adjustment arrangement comprises a feedbackamplifier configured to receive the second input to the amplifier of theanalog subtractor circuit.
 13. The current-to-voltage signal converterof claim 12, wherein the feedback amplifier is electrically coupled to afirst terminal of a first common mode resistor and to a first terminalof a second common mode resistor, wherein the second terminals of firstand second common mode resistors are electrically coupled to the firstand second input respectively, wherein the first and second common moderesistors have substantially the same value resistance.
 14. Thecurrent-to-voltage signal converter of claim 12, wherein the feedbackamplifier is configured to operate a current mirror arrangementcomprising two mirrored transistors and a control transistor, whereinthe feedback amplifier is arranged to control the current flow in thecontrol transistor and the two mirrored transistors are electricallycoupled to the first and second input respectively, wherein the twomirrored transistors are size matched.
 15. A transimpedance amplifier,comprising: a current-to-voltage converter, configured to receive afirst current signal, and to generate a positive voltage output; avoltage adjustment arrangement, coupled to the current-to-voltageconverter, and configured to enable at least part of thecurrent-to-voltage converter to operate between a first predeterminedvoltage threshold and a second predetermined voltage threshold.
 16. Thetransimpedance amplifier of claim 15, wherein the voltage adjustmentarrangement comprises a constant voltage/current source coupled to afirst terminal of a resistor, and a second terminal of the resistor iscoupled to the current-to-voltage converter.
 17. The transimpedanceamplifier of claim 15, wherein the voltage adjustment arrangementcomprises a feedback amplifier arranged to actively control the voltageadjustment based on the positive voltage output.
 18. A method ofconverting a current signal to a voltage signal, the method comprising:converting an input current signal to a first voltage signal using afirst stage of a signal converter; converting the first voltage signalinto a second voltage signal using an analog subtractor circuitcomprising an amplifier; and, applying a voltage adjustment arrangementto at least one input of the amplifier to reduce the voltage at the atleast one input to the amplifier between a first predetermined voltagethreshold and a second predetermined voltage threshold.
 19. The methodof claim 18, wherein the at least one input of the amplifier comprises afirst input and a second input, wherein the voltage adjustmentarrangement comprises a common mode reference coupled to both the firstinput and the second input, wherein the common mode reference is a DCsource, and further performing the steps of: predetermining a DC offsetvoltage of the first voltage signal; and predetermining a voltage orcurrent value of the DC source based on the predetermined DC offsetvoltage.
 20. The method of claim 18, wherein the at least one input ofthe amplifier comprises a first input and a second input, wherein thevoltage adjustment arrangement comprises a common mode reference coupledto both the first input and the second input, wherein the common modereference is a DC source, and further performing the steps of: adjustinga voltage or current value at both the first input and the second inputbased on a DC offset voltage of the first voltage signal.